This invention relates to an electromagnetic identification system of a kind which can be employed, for example, in electronic article surveillance (E.A.S.), as it may be used for in-store security, in access control, or in other applications such as the protection of documents or data storage devices at a place of work.
In any environment, an element is used which influences an electromagnetic field, so causing a disturbance in a characteristic of the field as detected by a detector device. The element is frequently referred to as a "tag" and needs to exhibit a number of characteristics one of which, especially relevant for in-store usage, is the capability of being readily de-activated by check-out personnel whilst being difficult for a would-be thief to de-activate.
It is usual for the element, or tag, to contain a strip of highly permeable magnetic material. Such material is easily influenced by a relatively weak interrogating field, causing the generation of a number of harmonic frequencies which are readily detected by a suitable detection device. Unless it is proposed to physically fracture or mechanically strain the strip in order to de-activate it, and such activities would place severe limitations upon the form which a tag could take, de-activation is usually achieved by magnetising a control element consisting of a normally unmagnetised, magnetically hard material disposed in the tag and configured so as to produce a series of poles along the length of the strip of highly permeable material when so magnetised. This alters the magnetic "profile" of the tag, as presented to the interrogating field, and inhibits or characteristically alters the generation of the aforementioned harmonics, thus permitting the detection device to distinguish between activated and de-activated tags.
It has been usual hitherto for the magnetically hard material constituting the control element to be provided either in the form of discontinuous lengths running close to the strip of magnetically soft material, or in continuous lengths similarly disposed. In the first case, de-activation is relatively straightforward for check-out personnel to accomplish, but the same can be said for would-be thieves as a single swipe along the tab with a permanent magnet of appropriate strength is all that is required to magnetise the magnetically hard material. In the second case, more care has to be taken by the check-out personnel because the continuous length of magnetically hard material has to be selectively magnetised to produce a pole pattern sufficient to change the electromagnetic profile of the tag, but at the same time de-activation is made more difficult for the would-be thief. It is an object of the present invention to provide an improved tag.